PROJECT SUMMARY/ABTRACT Drug overdoses are now the leading cause of injury death in the United States, accounting for near nearly half a million deaths between 2000 and 2014. Over this time, opioid overdose deaths have risen at a higher rate than overdose deaths associated with other drugs, suggesting opioids are largely driving increases in mortality. In addition to the devastating human costs of overdoses to families and communities, the economic burden of prescription opioid overdoses is estimated at approximately $80 billion a year. Pharmacologic treatments for opioid use disorder are effective at reducing illicit opioid use but are vastly underutilized. There are three pharmacological treatments for opioid use disorder: methadone, buprenorphine, and naltrexone. Buprenorphine is a promising option for increasing access to treatment for opioid use disorder. Unlike methadone, buprenorphine can be prescribed by office-based physicians, and unlike naltrexone, buprenorphine can be prescribed to individuals actively using opioids. Medicaid-enrolled individuals are at higher risk of opioid overdose, and Medicaid is the largest single payer of substance use services. Nevertheless, there is limited evidence concerning access and utilization of buprenorphine treatment in Medicaid programs. The few existing studies suggest that recent health reforms have not increased substance use service utilization and few buprenorphine providers treat Medicaid patients. The overall objective of this Kirschstein-NRSA F30 fellowship is to generate evidence that can guide policies to improve buprenorphine utilization and access in Medicaid. The specific aims of this proposal are: 1) To evaluate the effects of Medicaid expansion on overall state-level utilization of buprenorphine; and 2) To describe insurance-related barriers to buprenorphine provision among providers. This proposed research includes a combination of quantitative and qualitative research methods: quasi- experimental methods to evaluate policy effects (Aim 1) and semi-structure interviews with providers on treatment barriers (Aim 2). Through this research project and a carefully constructed training plan, the trainee will achieve the following fellowship goals: 1) Develop advanced skills in research using quasi-experimental design; 2) Develop skills in mixed methods research data collection, analysis, and interpretation; 3) Gain knowledge of theories, methods, and issues in implementation science; and 4) Develop clinical and professional skills for a career as an academic physician-scientist.